A wheel is a unit of length measurement in Mid-World. In The Dark Tower VII: The Dark Tower it is stated that 12 wheels is about 10 miles or somewhat less, suggesting that 1 wheel is approximately 8/10 of a mile.
In The Wind Through the Keyhole, the Daria portable guidance module states that 50 miles is 45.45 wheels, suggesting that a wheel is 4800 feet.